1. Field of the Invention
The present invention relates to liquid crystal substrates and reflective-type liquid crystal panels using the substrate, and in particular, relates to a technique that can preferably be applied to active-matrix liquid crystal panels in which pixel electrodes are switched by switching elements formed on the substrate. Furthermore, the present invention relates to electronic equipment and projection type display devices both using the liquid crystal panel.
2. Background of the Related Art
Conventionally, as active-matrix liquid crystal panels used for light valves of projection type display devices, liquid crystal panels having a thin film transistor (TFT) array, employing amorphous silicon, on a glass substrate as switching elements of pixels have been put into practical use.
Active-matrix liquid crystal panels using the above TFTs have low TFT element mobility and a large device size. Thus, for example, a projection type display device, such as a projector, equipped with the liquid crystal panel as a light valve, is disadvantageously large in size. Furthermore, transmissive-type liquid crystal panels have the following fatal disadvantage: the aperture ratio decreases as the resolution of the panel increases, such as XGA or SXGA, since the regions of the TFTs provided for every pixel do not transmit light.
As compared with the transmissive-type active-matrix liquid crystal panels, reflective-type active-matrix liquid crystal panels are small in size and have an insulated gate field effect transistor (MOSFET) array formed as switching elements on a semiconductor substrate so as to control the voltage applied to pixel electrodes which are to be used as reflective electrodes.
As is mentioned above, in active-matrix liquid crystal panels having transistor elements formed on a glass or semiconductor substrate, when light leaks through spaces formed between the pixel electrodes, hole-electron pairs are generated in a PN junction (e.g., a junction between source/drain regions and a channel region of the transistor, or a junction between source/drain regions and a well) of the semiconductor layer or semiconductor substrate, so that a light leakage current flows and undesirably destabilizes the electric potential of the semiconductor layer, the semiconductor substrate, or the well. In the case of reflective-type liquid crystal panels, the amount of light leakage can be reduced as compared with that of the transmissive type by, for example, forming the pixel electrodes close to each other in the top layer without using particular light-shielding means. However, in reflective-type liquid crystal panels used for light valves of projection type display devices, strong light is converged and is incident on the spaces between the pixel electrodes. Thus, it is not sufficient to arrange the pixel electrodes close to each other to avoid the light leakage current.
In particular, since liquid crystal panels with a semiconductor substrate have well regions, the leaking light transmitted through not only the transistor portion but also the portion at a certain distance from the transistor portion may cause a light leakage current. Therefore, unless sufficient countermeasures are taken, the light leakage current increases as compared with liquid crystal panels having TFTs as switching elements on a glass substrate.
Furthermore, in active-matrix liquid crystal panels having transistor elements on a glass or semiconductor substrate, peripheral circuits such as a scanning side driving circuit and a data line driving circuit are formed on the same substrate; there is a problem such that the light leakage current is generated and the peripheral circuits are operated by mistake when light enters to such peripheral circuits.
Moreover, in reflective-type liquid crystal panels, an insulating film is exposed by the spaces between the pixel electrodes, and the light reflected by the surface of the insulating film changes its direction by 180xc2x0 and emerges. As a result, the emerging light is displayed as unwanted light, which deteriorates the quality of the image.
It is therefore an object of the invention to provide a technique by which the amount of light leaking through the spaces between pixel electrodes is reduced so that the light leakage current generated in a substrate decreases.
Another object of the present invention is to provide a technique for reducing the amount of light leaking into the pixel region and peripheral circuits without increasing the number of the process steps in reflective-type liquid crystal panels in which pixel electrodes are arranged in a matrix pattern and peripheral circuits are provided outside the pixel region.
Still another object of the present invention is to provide a technique for preventing adverse effects on image quality due to the light reflected by the surface of an insulating film exposed to the spaces between the pixel electrodes in reflective-type liquid crystal panels.
The first through eighteenth aspects of the invention are discussed below.
First, a liquid crystal panel substrate comprises: pixel units each having a pixel electrode to be used as a reflective electrode and arranged according to a matrix pattern on a substrate; and a switching element controlling a voltage applied to the pixel electrode;
in which between the pixel electrode and a conductive layer comprising a terminal electrode of the switching element, a contact hole is made for connecting the pixel electrode and the terminal electrode; and
a light-shielding layer, having an opening surrounding the portion in which the contact hole is formed, and having no opening in regions between a plurality of adjacent pixel electrodes, is formed between the pixel electrode and the conductive layer. The amount of light leaking through the space between the pixel electrodes and reaching the switching element can thereby be reduced to substantially zero.
Secondly, an anti-reflection film is provided between the pixel electrode and the light-shielding layer. The light which is incident on the space between the pixel electrodes and reflected by the surface of the light-shielding layer can thereby be absorbed even when the light-shielding layer is formed of a metallic layer having a relatively high reflectance, such as aluminum.
Thirdly, the anti-reflection film has substantially the same shape as that of the pixel electrode and is provided below the pixel electrode. Thus, the following phenomenon can be prevented: the light, which is incident on the space between the pixel electrodes, is repeatedly reflected between the surface of the light-shielding layer and the back surface of the pixel electrodes, leaks through an opening provided at the portion of a connecting conductor connecting a pixel electrode and a switching electrode, reaches a semiconductor layer or a semiconductor substrate, and generates a light leakage current.
Fourthly, the anti-reflection film is made of titanium nitride. Titanium nitride has excellent adhesion to the pixel electrode such as Al and has excellent light absorbance.
Fifthly, the film thickness of the titanium nitride is 500 to 1000 angstroms. This range is preferable to absorb visible light.
Sixthly, in regions between a plurality of adjacent pixel electrodes, a groove at least having a slope is formed on the surface of an underlying insulating layer of the pixel electrode or on the surface of the light-shielding layer under the underlying insulating layer. The light entering through a space between the pixel electrodes can thereby be reflected in an oblique direction. Thus, the following phenomenon can be prevented: the light incident on the space is reflected by the light-shielding layer, emerges through the space, and is mixed with the light reflected by the pixel electrodes. As a result, the contrast of the liquid crystal panel can be improved.
Seventhly, the anti-reflection film has substantially the same shape as that of the pixel electrode and is provided below the pixel electrode. The light reflected by the surface of the insulating film, exposed by spaces between the pixel electrodes, or the light-shielding layer below the insulating film, is thereby absorbed into the antireflection film formed on the back surface of the pixel electrodes so that the light is prevented from emerging while changing its direction is changed by 180xc2x0, resulting in improved image quality.
Eighthly, the anti-reflection film is made of titanium nitride.
Ninethly, the film thickness of the titanium nitride is 500 to 1000 angstroms. The effects thereof are similar to those of the fourth and fifth effects.
Tenthly, the contact hole is provided at a substantially central position of the plane of the pixel electrode. The distance between the end portion of the pixel electrode and the opening provided in the light-shielding layer is therefore substantially the same for each end portion. Thus, the travel distance of the light, entering through a space between the adjacent electrodes and reaching the contact hole, is increased and the light cannot readily reach the switching element side.
Eleventhly, a liquid crystal panel substrate comprises pixel units which are arranged in a matrix pattern on a substrate, and each of which has a pixel electrode to be used as a reflective electrode and a switching element controlling a voltage applied to the pixel electrode;
wherein a pixel region comprising a plurality of the pixel units and a peripheral circuit provided at a peripheral region of the pixel region are formed on the same substrate; and
a light-shielding layer comprising the same layer as the reflective electrode of the pixel region is formed above the peripheral circuit. The amount of light leakage in the pixel region and the peripheral circuit can thereby be reduced, without increasing the number of steps for producing the liquid crystal panel substrate.
Twelvethly, the light-shielding layer is positioned at a peripheral region surrounding the entire periphery of the pixel region including a region in which the peripheral circuit is not formed. The pixel electrodes are positioned around the pixel region and serve as xe2x80x9cpartitionsxe2x80x9d.
Thirteenthly, in the pixel region, a second light-shielding layer is provided between the pixel electrode and the switching element, and the second light-shielding layer is also provided in a region between the first light-shielding layer, which is provided above the peripheral circuit, and the pixel electrodes that are outermost in the pixel region. In the pixel region, the entrance of light from the border between the pixel region and the peripheral circuit region can be prevented by providing the second light-shielding layer under the pixel electrode.
Fourteenthly, between the pixel electrode and a conductive layer comprising a terminal electrode of the switching element, a contact hole is made for connecting the pixel electrode and the terminal electrode;
a second light-shielding layer, having an opening surrounding the portion in which the contact hole is formed in the pixel region and having no opening in regions between a plurality of adjacent pixel electrodes, is formed between the pixel electrode and the conductive layer; and
in the peripheral circuit, the second light-shielding layer is also provided below the first light-shielding layer and is used as a connecting line portion in the peripheral circuit. Thus, multi-layer wiring can be achieved in the peripheral circuit portion by utilizing the light-shielding layer in the pixel region, and the driving circuit and the like can be integrated.
Fifteenthly, the second light-shielding layer has a light-shielding portion that extends or is separated from the connecting line portion. The peripheral circuit can thereby be protected from light by double-layered light-shielding layers.
Sixteenthly, the present invention can provide a liquid crystal panel which suppresses a reduction in contrast due to the light leakage current, and which comprises: a liquid crystal panel substrate according to the above-mentioned present invention; a light-incident-side substrate positioned opposing the liquid crystal panel substrate with a space therebetween; and a liquid crystal filled in the space.
Seventeenthly, the present invention can provide electronic equipment which is equipped with the above liquid crystal panel as a display portion, and which have a reflective type display device having excellent contrast at low electric power consumption.
Eighteenthly, the present invention can provide a small-sized projection type display device which has excellent contrast, and which comprises: a light source; the above liquid crystal panel reflecting and modulating the light emerging from the light source; and a projection optical means for collecting and projecting the light modulated by the liquid crystal panel.